South Korean police are investigating 17 US soldiers and five other people who allegedly smuggled or used synthetic marijuana via military mail.
This follows raids in at least two US army bases in May, including Camp Humphreys, its largest overseas base.
A Filipino and a South Korean have been arrested, while prosecutors review the cases against all 22 suspects.
A tip from the US Army’s enforcement arm had sparked a four-month investigation by Korean authorities.
It was one of the largest in recent years involving American soldiers, US media reported citing Cha Min-seok, a senior detective in South Korea.